This invention relates to switching circuits and more particularly to a constant current drive for a switching power supply.
Switching circuits contained within a switching power supply use switching transistors. The switching transistors switch current bidirectionally through a high frequency step down transformer. The current is supplied by a high voltage rectified and filtered signal from an input transformer. The switching transistors switch the current at a relatively high frequency in the order of tens of kilohertz. The high frequency step down transformer is physically smaller than low frequency transformers and may contain multiple taps on its secondary windings. The taps provide multiple voltages that are to be rectified, filtered, regulated and distributed to various logic modules within an electronic system.
The switching circuits include an input transistor switching stage, a transformer coupling stage and an output transistor switching stage. The output transistor switching stage drives a primary winding of the high frequency step down transformer. The primary winding of the high frequency step down transformer carries bidirectional current received from a high frequency high voltage input signal. The coupling transformer stage is used in a voltage coupling mode and couples a voltage level onto the output switching transistors of the output switching transistor stage. The coupled voltage onto the output switching stage is coupled across a series connected base resistor and a base-emitter junction of an output transistor. Because of the variable base-emitter resistance, the base current of the output transistor switching stage is difficult to control and consequently, a resulting collector current is then difficult to control as is the resulting transistor current into the high frequency step down transformer.
However, a series base resistor is used to solve or minimize these problems by desensitizing the output transistor current to the variable base-emitter resistance. The resistance of the base resistor used is much greater than the base-emitter resistance, and it is not variable and thereby dominates the base-emitter resistance. The use of such a base resistor thus controls the base current drive. However, the use of a base resistor in a series connection to the base of the transistor causes large power dissipation to occur in the base resistor. The power loss disadvantageously decreases the efficiency of the power supply.
The voltage drive to the high frequency step down transformer will decay to zero or increases exponentially to the supply voltage when the voltage drive to an output transistor is turned off or on, respectively. Slow turn-on and slow turn-off times cause the output switching transistors to dissipate large amounts of power during switching. This power dissipation disadvantageously decreases the efficiency of the switching power supplies.
A standard design of the input transistor stage comprises NPN transistors with the emitters thereof grounded. The collectors are connected to the coupling transformer. Logic control signals applied to the bases will cause their respective transistors to conduct. A resulting voltage is thereby applied across the primary windings of the coupling transformers. The collector of each of the input transistors is connected to the respective ends of the primary windings of the coupling transformers. The center tap of each of the primary windings is connected to a voltage source. This voltage source that is connected to the center taps of the primary windings of the coupling transformers, is usually a regulated power source which provides a controlled voltage across the secondary windings and the base resistors and base emitter junction of each of their respective output switching transistors. Thus, traditional switching circuits in switching power supplies disadvantageously use an expensive regulated voltage source to control the voltage applied to output switching transistors.